Overall objective of the proposed research for the next grant period will continue to be: (1) to develop efficient, highly stereocontrolled methods for the synthesis of (a) steroid side chains and (b) the C/D- ring trans-hydrindanes with fuunctionalized side chain characteristic of physiologically important steroids. These include vitamin D3 and its active metabolites, pregnenolone, cholesterol metabolites, insect molting hormones, and some plant hormones; (2) as an extension of the first objective, to explore the general applicability of the developed methodologies towards the stereocontrolled synthesis of acyclic carbon chain systems. We propose the following synthetic endeavors as specific aims towards the general objectives stated above: (1) synthesis of functionalized trans-hydrindanes with side chains or their appropriate precursors which are amenable to the synthesis of vitamin D3 metabolites, pregnenolone, 11-keto steroids such as adrenosterone, (2) synthesis of 24(28)-Z- and E-24, 28-didehydrooogoniols for examination of biological activity, (3) use of the 6-endo radical-mediated cyclization as a means of chirality transmission in the stereocontrolled synthesis of the plant hormone brassinolides, the insect pheromone invictolide and other natural 1,3,5- trimethylated carbon chains which can be used as building blocks of the antibiotic macrolides, (4) use of [2,3]-Wittig and [3,3-Claisen rearrangements for the stereocontrolled synthesis of the 1,3,5- trimethylated carbon chains and (5) development of the new variants of sigmatropic rearrangements and their applications to the stereocontrolled synthesis of biologically active natural products.